Fidaxomycin, 3-(((6-Deoxy-4-O-(3,5-dichloro-2-ethyl-4,6-dihydroxybenzoyl)-2-O-methyl-β-D-mannopyranosyl)oxy)-methyl)-12(R)-[(6-deoxy-5-C-methyl-4-O-(2-methyl-1-oxopropyl)-b-D-lyxo-hexopyranosyl)oxy]-11(S)-ethyl-8(S)-hydroxy-18(S)-(1(R)-hydroxyethyl)-9,13,15-trimethyloxacyclooctadeca-3,5,9,13,15-pentaene-2-one, (IUPAC), has the following chemical structure:

Fidaxomycin, formerly known as OPT 80, PAR 01, PAR 101, R-Tiacumicin B, Tiacumicin B, Lipiarmicin, Lipiarmycin, and Lipiarmycin A3, is a naturally occurring 18-membered macrocycle, which is derived from the fermentation of Dactylosporangium aurantiacum subspecies hamdenesis. 
Fidaxomycin is being developed by Optimer Pharmaceuticals as an oral, narrow-spectrum antibacterial. In particular, Fidaxomycin shows activity against Clostridium difficile. 
Fidaxomycin, and a process for its preparation, is disclosed in Journal of Antibiotics, vol. XL, no. 5, pages 575-588 (1987). Several solid state forms of mixtures containing Fidaxomycin as well as other tiacumicins are disclosed in U.S. Pat. No. 7,378,508 and its European counterpart EP 2 125 850 B1. In particular, two polymorphic forms termed form A and form B of “Fidaxomycin” (containing varying amounts of structural analogs) are described, and the applicant stated during prosecution that the claimed hydrated form A exhibits different thermodynamic properties and is more stable compared to the ethyl acetate solvate form B. The solid state forms of Fidaxomycin described therein were prepared by a procedure which required about 3 to about 14 days. In any event, the polymorphic forms described in these patents are comprised of mixtures of Fidaxomycin with up to 15% or more of other, structurally related tiacumicins.
Polymorphism, the occurrence of different crystal forms, is a property of some molecules and molecular complexes. A single molecule, like Fidaxomycin, may give rise to a variety of polymorphs having distinct crystal structures and physical properties like melting point, thermal behaviors (e.g. measured by thermogravimetric analysis—“TGA”, or differential scanning calorimetry—“DSC”), powder X-ray diffraction (PXRD) pattern, infrared absorption fingerprint, and solid state NMR spectrum. One or more of these techniques may be used to distinguish different polymorphic forms of a compound.
Different solid state forms (including solvated forms) of an active pharmaceutical ingredient may possess different properties. Such variations in the properties of different solid state forms and solvates may provide a basis for improving formulation, for example, by facilitating better processing or handling characteristics, changing the dissolution profile in a favorable direction, or improving stability (polymorph as well as chemical stability) and shelf-life. These variations in the properties of different solid state forms may also offer improvements to the final dosage form, for instance, if they serve to improve bioavailability. Different solid state forms and solvates of an active pharmaceutical ingredient may also give rise to a variety of polymorphs or crystalline forms, which may in turn provide additional opportunities to assess variations in the properties and characteristics of a solid active pharmaceutical ingredient.
Discovering new polymorphic forms and solvates of a pharmaceutical product can provide materials having desirable processing properties, such as ease of handling, ease of processing, low hygroscopicity, storage stability, and ease of purification or as desirable intermediate crystal forms that facilitate conversion to other polymorphic forms. New polymorphic forms and solvates of a pharmaceutically useful compound or salts thereof can also provide an opportunity to improve the performance characteristics of a pharmaceutical product (dissolution profile, bioavailability, etc.). It enlarges the repertoire of materials that a formulation scientist has available for formulation optimization, for example by providing a product with different properties, e.g., different crystal habits, higher crystallinity or polymorphic stability which may offer better processing or handling characteristics, improved dissolution profile, or improved shelf-life. Lastly, new polymorphic forms may be prepared with improved reliability and reproducibility in manufacturing and processing compared to other forms, for example, in terms of crystallinity or polymorphic purity.